256 Prof. Tyndall on Recent Researches on Radiant Heat. 



the combustion of phosphorus in air differed from both ; while 

 the nitrogen obtained from the nitrate of potassa could not 

 be made to agree with its fellows. In like manner, the 

 oxygen obtained from the chlorate of potash and peroxide of 

 manganese differed from electrolytic oxygen ; the hydrogen 

 obtained from sulphuric acid and zinc differed from electrolytic 

 hydrogen ; the carbonic oxide obtained from chalk and carbon 

 differed from that generated from the ferrocyanide of potas- 

 sium, while carbonic acid from different sources showed similar 

 anomalies. It will be borne in mind that at this time nothing 

 whatever was known of the vast action which a small amount of 

 certain impurities can exert, and that my own experiments were 

 the first to exhibit this action. 



Further, my drying apparatus first consisted of sixteen feet of 

 glass tubing filled with chloride of calcium, and a large U-tube 

 filled with fragments of pumice-stone moistened with sulphuric 

 acid. Sometimes the chloride of calcium was used alone, some- 

 times the sulphuric acid, and sometimes both were used together. 

 Every morning it was necessary to allow the air to pass through 

 the drying apparatus, and fill the experimental tube several times 

 before the results became constant; and even after they had 

 become tolerably constant with the chloride of calcium, the 

 introduction of the sulphuric acid caused a considerable variation 

 of the absorption. This might naturally be ascribed to the 

 more perfect desiccation of the air by the acid, but this does not 

 account for the effects which I obtained. For when both were 

 used, the magnitude of the absorption was found to depend on 

 the circumstance whether the air entered the sulphuric-acid tube 

 or the chloride- of-calcium tube first, I will here give an example 

 of this irregularity. 



Absorption. 



Air passed through Ca CI alone 7 



"When SO 3 was added 4 



Through new Ca CI tube ........ 7 



New SO 3 tube added 4 



Through another Ca CI tube alone ..... 7 



A fresh tube of SO 3 added 5 



Reversed current of air, and sent it through SO 3 



first 10 



The fluctuations above referred to are here distinctly exhibited ; 

 and the last experiment shows that, without changing the tubes 

 in any way, but merely by reversing the direction in which the 

 eurrent of air passed through them, the absorption was doubled. 

 Difficulties almost innumerable of this kind had to be overcome. 

 I had finally to abandon the chloride of calcium and the pumice- 

 stone altogether, and make use of fragments of pure marble for 



